(1) Field of the Invention
The invention relates generally to electronic circuits for the evaluation of the physical effects on capacitive components and more particularly, to circuits for detecting capacitance variations at capacitive sensors or transducers and even more particularly to a circuit for the detection of the x-, y- and z-coordinate position changes measured with displacement-responsive devices such as joysticks built with variable capacitors. Both, a circuit and a method are disclosed.
(2) Description of the Prior Art
Many electronic sensoring devices on the market today are built by eploiting capacitive effects by some means or other. Amongst them there are devices for touch or actuator buttons e.g. implementing a non-mechanical switch as used for example in elevator control panels or vandal-proof telephone dialers. Further on high reliability switches for keyboards are manufactured with such capacitive contacts normally used in computer systems or any other industrial equipment e.g. tooling machinery, but also in control devices for commercial and consumer products e.g. television sets or video recorders. In general any proximity sensors using the change of the dielectric constant of a dedicated capacitor element fall into that category, e.g. changed by a nearing body such as a hand or finger. Sensors for detecting humidity or even measuring moisture are also using that principle. Numerous devices are available or have been proposed for use as object position detectors for use with computerized systems and other applications equally based on capacitive change measurements. Maybe the most important applications are however found in the field of non-contact displacement measurements as adopted in capacitive displacement transducers, which are known for use in displacement-responsive devices such as measurement probes and joysticks, where a stylus or lever is movable in the directions of two or more orthogonal axes and thus altering the values of the correspondent capacitors in its base. These devices become even more important in the future as electronic control and steering is further finding its way into terrestrial, maritime and airborne vehicles of any kind, where joystick solutions are willingly adopted. Normally there would be one or more separate capacitive transducers for each axis of movement. Each such transducer comprises at least a pair of capacitor plates which are relatively movable. In some known devices there are three or more capacitor plates in each transducer, so that the transducer comprises a differential pair of capacitors. Such an ingenious arrangement has improved performance compared to single capacitor solutions. The needed electronic evaluation circuits however turn out to be rather complicated and have to be cleverly devised.
While such high reliability and high stability devices—also used in the implementation of this invention—offer promise in certain sophisticated applications, price performance issues continue to limit their desirability. Further, many prior art devices use elaborate analog signal processing for a substantial portion of the circuitry, which imposes further drawbacks.
There has therefore been a need for a capacitive sensing device which performs sensing simply, inexpensively and rapidly, while at the same time avoiding problems with temperature, moisture and pressure variation. There has also been a need for such a sensing device which facilitates conversion to digital processing as quickly as possible, as well as permitting a higher degree of integration than has previously been possible.
It is therefore a challenge for the designer of such methods and circuits to achieve a high-quality and low-cost solution. Several prior art inventions referring to such solutions describe related technologies, methods and circuits.
U.S. Pat. No. 5,790,107 (to Kasser et al.) describes a method and an apparatus for determining small variations in capacitance, typically in a capacitive touch sensing device, including a reference signal of a first frequency and a sample signal of a second frequency. The reference and sample signals are mixed and filtered to isolate the beat frequency therebetween, which is then measured to provide an indication of the variation in the sample frequency. The measurement can then be manipulated by a microprocessor or microcontroller to provide desired control signals. The apparatus is susceptible of digital implementation and single chip implementation.
U.S. Pat. No. 5,374,787 (to Miller et al.) discloses a proximity sensor system including a sensor matrix array having a characteristic capacitance on horizontal and vertical conductors connected to sensor pads. The capacitance changes as a function of the proximity of an object or objects to the sensor matrix. The change in capacitance of each node in both the X and Y directions of the matrix due to the approach of an object is converted to a set of voltages in the X and Y directions. These voltages are processed by analog circuitry to develop electrical signals representative of the centroid of the profile of the object, i.e., its position in the X and Y dimensions. The profile of position may also be integrated to provide Z-axis (pressure) information.
U.S. Pat. No. 5,006,952 (to Thomas) shows a signal conditioning circuit for multiple channel capacitive displacement transducers. The transducer comprises three differential capacitive transducers, driven by respective square waves from modulators. The modulators are driven at different frequencies which are even multiples of each other, derived from a divider. The signals from each differential capacitance pair of the transducer are received by a common charge amplifier. They are then demodulated by respective phase sensitive rectifiers, each of which is driven at the same frequency as the corresponding modulator. The outputs of the phase sensitive rectifiers are integrated, e.g. by circuits for one of the channels, and the resulting voltage signal is fed back to the corresponding modulator so as to tend to null the input to the charge amplifier. The use of modulating frequencies which are even multiples of each other enables the signals to be multiplexed through a single charge amplifier. The mechanical construction of the multi-channel transducer is also described.
Although these patents describe circuits and methods close to the field of the invention they differ in essential features from the method and especially the circuit introduced here.